it is possible that yo have two installations on you system, and TeXmaker only finds the one where apa6.cls is not installed. Having typed this, i notice that you reported that package apa6 is missing, which is indeed the case as it it nonexistent (or somebody created the file and i don't know about it).

Concerning the bibliography, i recommend using the biblatex package biblatex-apa. There are hints in the documentation on how to set the bibliography (and every single entry if needed) localized, meaning y (es), et (fr), und (de), and (en) and finalle &.

I don't know anything about your current setup, so i cannot give more detailed hints. Please read about creating minimal working examples. They not only give helpers just the stuff to solve a problem, but also save a lot of time, meaning: the chance of getting a quicker and better answer raises.

The smart way: Calm down and take a deep breath, read posts and provided links attentively, try to understand and ask if necessary.